"The sectarian brutality is changing the country's demography," Ban Ki-moon told reporters yesterday.
"The de facto partition of the C.A.R is a distinct risk."
The impoverished former French colony descended into chaos last March after rebels overthrew the government, sparking deadly Christian-Muslim violence that has uprooted a million people.
"The international response does not yet match the gravity of the situation," Ban said.
"We must do more to prevent more atrocities, protect civilians, restore law and order, provide humanitarian assistance and hold the country together."
France has deployed 1,600 troops, while the United States is providing logistical support.
The European Union, meanwhile, has promised to deploy 500 troops to the capital Bangui at the beginning of March.
"The international response must be robust - with a credible deployment of force," he said.
"I ... Continue to urge the international community to support the AU and MISCA at this time," he added as he called on the EU to "accelerate the deployment of its military operation."
Ban said he would follow up with Fabius later yesterday.
Edmond Mulet, the UN's assistant secretary general for peacekeeping operations, would travel to the Central African Republic this week to consult with AU representatives on possibly transforming MISCA into a UN peacekeeping operation, Ban said.
"But even if that change looks increasingly necessary, it would take time for it to happen," he added.
Also yesterday, French Defence Minister Jean-Yves Le Drian called on the deployed international forces to put an end to brutal attacks by the country's militias - "by force if needed.